System for monitoring damage to a rotor blade of a rotary-wing aircraft

ABSTRACT

The present invention relates to a rotary-wing aircraft rotor blade ( 23 ) that presents a longitudinal axis ( 26 ) and that includes (semi)conductive loops ( 10 ) extending along axes ( 19, 19 A to  19 C,  19 W to  19 Z) or directions that are oblique relative to the longitudinal axis ( 26 ).

The present invention relates to a system for monitoring damage to arotor blade of a rotary-wing aircraft, to an aircraft rotor blade and toan aircraft fitted with such a system.

The technical field of the invention is that of manufacturingrotary-wing aircraft.

Various devices for monitoring damage to a blade of a rotor forproviding a helicopter with lift and forward drive have been proposed.

Patents FR 2 119 182 and U.S. Pat. No. 3,744,300 describe a device formeasuring damage due to fatigue in a helicopter blade, the deviceincluding a sensor comprising strain gauges connected in a Wheatstonebridge configuration.

U.S. Pat. No. 4,106,332 describes a device for detecting an interruptionin a conductive loop applied to the surface of a helicopter rotor blade;the device includes a bistable serving, in flight, to record a statecorresponding to detecting a fault that led to the conductive loop beinginterrupted; when the aircraft is on the ground, a unit can be connectedto the blade in order to read the state of the bistable in order to testthe blade while it is at rest.

A drawback with such systems is that they do not make it possible todetermine the location and the extent of the fault affecting the blade.

U.S. Pat. No. 3,596,269 describes a system for detecting and monitoringstructural faults in the skin of an airplane. That system comprises agroup of thin elongate electrical conductors constituted by two metalsheets interconnected by a resistor; the conductors are connected inparallel and they are placed between two insulating sheets; the assemblyis stuck to different points on the skin of an airplane. A switchenables an operator manually to select a group for measuring itsresistance by means of an ohmmeter; a visual or audible alarm isprovided.

A drawback with such systems is that they are adapted poorly or not atall to monitoring the defects that might appear on the surface or insidea blade made of composite material.

U.S. Pat. No. 4,524,620 describes apparatus for acoustically monitoringthe noise emitted by a composite helicopter blade; the blade is fittedwith one or more acoustic transducers and the signals therefrom aremultiplexed where necessary, and then filtered and forwarded to acomputer or to a recorder; a filter serves to detect and eliminate“normal” noise so as to pass only “critical” noise representative ofexcessive stresses in the blade and structural damage caused to theblade; the occurrence and the intensity of such critical noise areaccumulated in the recorder or the computer in order to constitute ahistory of critical noise from a given blade.

Such a device is complex; furthermore locating a defect in the bladerequires triangulation calculations to be performed, in deferred time,and on the ground, using an auxiliary computer.

An object of the invention is to propose a rotary wing blade ofcomposite material that is fitted with a simple and high-performancesystem for monitoring damage to the structure of the blade.

An object of the invention is to propose a system for acting while arotary-wing aircraft is flying to monitor the appearance and thebehavior in real time of damage to one or more blades of the wing, whichsystem is improved and/or remedies the shortcomings and drawbacks ofknown systems, at least in part.

An object of the invention is also to propose a rotary-wing aircraftincluding such a monitoring system.

In a first aspect of the invention, there is provided a rotor blade fora rotary-wing aircraft, the blade presenting a longitudinal axis andincluding (semi)conductive loops extending along axes or directions thatare oblique relative to the longitudinal axis.

In a preferred embodiment of the invention, a blade is provided for amain or auxiliary rotor of a rotary-wing aircraft, the blade comprisinga root, a leading edge, a trailing edge, at least one spar, and a skincovering the spar(s); the blade includes a plurality of conductive orsemiconductive loops extending under the skin and disposed obliquely ortransversely relative to the longitudinal axis of the blade; the bladefurther includes a loop selector, and a member for measuring theresistance (or the impedance) of a loop, which selector and member areintegrated in the root of the blade, and a plurality of electricalconnections connecting the selector to each of the loops, respectively;this enables data or impedance measurements concerning each of the loopsto be transmitted to a computer on board the aircraft while using onlytwo conductors, or where appropriate, via a single-channel wirelesslink.

In another preferred embodiment of the invention, there are provided: ablade for an aircraft rotary wing, the blade including a plurality ofloops each presenting an impedance; an impedance-measuring devicedesigned and arranged to measure the impedance of a loop, and deliveringa loop-impedance measurement signal; a scanner and connector devicedesigned and arranged to connect each of the loops in the plurality ofloops in succession to the impedance-measuring device, by scanning theloops of the plurality of loops regularly and in a determined order; ananalog-to-digital converter designed and arranged to transform eachmeasurement of the impedance-measuring device into digital impedancedata; and transmission means designed and arranged to transmit theimpedance data to an on-board computer.

In another aspect of the invention, there is a provided a blade for amain or auxiliary rotor of a rotary-wing aircraft, in particular ahelicopter, the blade presenting a composite structure including fibers,in particular glass or carbon fibers, that are embedded in athermoplastic or thermosetting matrix; the blade including a pluralityof substantially conductive loops (possibly including semiconductorelements, where appropriate) each presenting an impedance; the loopsextending parallel to some of the fibers so that the mechanicalperformance of the blade fitted with the loops is substantiallyidentical to that of the same blade not having any loops, wherebyinserting the loops in the structure of the blade does not significantlydegrade or modify the mechanical and aerodynamic characteristics of theblade, and whereby manufacture of the blade is also facilitated.

In other preferred embodiments of the invention:

-   -   the blade includes, on its pressure side and/or its suction        side, a first group of substantially conducive loops extending        in a first direction parallel to first fibers and/or to a first        sheet of fibers of the structure of the composite blade; the        blade further includes a second group of substantially        conductive loops extending in a second direction parallel to        second fibers and/or to a second sheet of fibers of the        structure of the composite blade, the second direction being        different from the first direction;    -   the blade includes a first array of first loops that are        mutually parallel and disposed in a first layer, and a second        array of second loops that are mutually parallel and that cross        the first array;    -   the blade includes a layer of electrically insulating material        separating two crossed and superposed arrays or layers of        conductive loops;    -   each loop includes a plurality of bridges or conductive or        semiconductive elements interconnecting the two strands of the        loop (and/or connected in parallel);    -   each of these bridges or elements presents an impedance (in        particular a resistance) that is substantially identical, e.g.        close to a few hundreds or thousands of ohms;    -   each loop includes at least three bridges or (semi)conductive        elements regularly spaced apart from one another along the axis        of the loop;    -   the blade includes several tens or hundreds of bridges or        (semi)conductive elements, and where appropriate several tens or        hundreds of loops including such bridges or (semi)conductive        elements;    -   the number of loops depends on the spacing or “pitch” between        loops, and determines the size of damage that can be measured;        this pitch may vary over the blade depending on the degree of        vulnerability of the zone being monitored;    -   said first and second directions along which the series of loops        extend are inclined relative to the longitudinal axis of the        blade and relative to a transverse axis of the blade (orthogonal        to the longitudinal axis), e.g. being inclined relative to these        axes by an angle close to 45° (to within plus or minus 10°);    -   said first direction is substantially perpendicular to said        second direction;    -   said loops are connected to the scanner or selector device by        respective pairs of metal wires (or strips);    -   said bridges or (semi)conductive elements are surface-mounted        components (SMCs);    -   the loops, the loop selector(s), the impedance-measuring        device(s), and where appropriate the analog-to-digital        converter(s) integrated in the blade are powered electrically by        a supply of electrical energy integrated in the blade (an        optionally-rechargeable battery), and/or by a circuit for        supplying power for de-icing the blade;    -   the signals or data relating to the impedances of the loops are        transmitted to an on-board calculator via a slip-ring assembly        connecting a rotor bearing to the rotor hub, and/or via wireless        transmission, e.g. radio transmission;    -   the conductive elements connecting the loops to the loop        selector are grouped together in a bundle or cord extending        close to the leading edge of the blade, the loops extending from        said bundle or cord towards the trailing edge of the blade.

In particular, the cord is preferably located inside or in the immediateproximity of the spar of the blade. This disposition enables the cord tobe protected by the spar which remains the last portion of the blade tobe damaged after an impact.

By using an oblique configuration for disposing the loops that can bemonitored for variations in impedance as a result of damage to a portionof the blade, it is possible to implement a system for monitoring thestate of the blade in flight that is simple, compact, and of highperformance.

In particular when the blade incorporates two crossed arrays of parallelloops, each loop including a plurality of same-impedance bridges, it ismuch easier to locate and evaluate damage that has caused electricalcontinuity to be broken in one or more loops.

Other characteristics and advantages of the invention appear from thefollowing description which refers to the accompanying drawings whichshow preferred and non-limiting embodiments of the invention.

FIG. 1 is an electrical circuit diagram of a loop having three bridgesin a system of the invention.

FIG. 2 is a simplified diagram of the FIG. 1 loop.

FIG. 3 is a diagrammatic plan view of a portion of a rotor blade showinghow two superposed crossed arrays of loops are installed that areidentical or similar to those of FIGS. 1 and 2.

FIG. 4 is a diagrammatic plan view of a helicopter rotor bladeincorporating a monitoring system of the invention.

FIG. 5 is a simplified block diagram of means for sequentially measuringthe impedances of the loops in a blade and for processing andtransmitting the measurements to a computer on board an aircraft.

FIG. 6 is a diagrammatic cross-section view of a blade of the invention.

With reference to FIG. 1 in particular, the loop 10 comprises twoparallel conductor strands 11 and 12 that are interconnected by threeidentical resistors 13, 14, and 15.

When the resistance of the three resistive bridges 13, 14, 15 isidentical and much greater than the resistance of the strands 11 and 12,then the impedance of the loop 10 as “seen” from its poles 16 and 17 isequal to one-third of the resistance of each of the bridges, assumingthere has been no damage; when damage to the blade causes at least oneof the strands 11, 12 to be broken in an end zone such as the zone 12 cat the end of the strand 12 (and of the loop 10), then the resistance ofthe loop becomes equal to half the resistance of each of the bridges;when damage to the blade breaks at least one of the strands 11, 12 in amiddle zone such as the middle zone 12 b of the strand 12 (and the loop10), then the resistance of the loop becomes equal to the resistance ofeach of the bridges; and when damage to the blade breaks at least one ofthe strands 11, 12 in a head zone such as the zone 12 a at the head ofthe strand 12 (and of the loop 10), then the resistance of the loopbecomes substantially infinite, since the loop is then open-circuit.

When the bridges 13 to 15 are regularly distributed along thelongitudinal axis 19 of the loop 10, the respective lengths 18 of thezones 12 a, 12 b, 12 c of the loop are equal.

With reference to FIGS. 2 and 3 in particular, in order to simplify thedrawings, and taking account of the fact that the length 20 of a loop 10is very long compared with the distance 21 between the strands 11 and 12of the loop, the bridges 13 to 15 are represented diagrammatically byblack spots tangential to the two parallel strands 11 and 12 of eachloop 10.

With reference to FIG. 3 in particular, the blade 23 extends along alongitudinal axis 26 and presents a leading edge 24 and a trailing edge25 that are substantially parallel to the axis 26.

The blade has a first array of loops comprising three substantiallyrectilinear loops 10 a, 10 b, and 10 c extending along respectivemutually parallel axes 19A, 19B, and 19C; these axes 19A to 19C are atan angle 22 close to 45° relative to the longitudinal axis 26 of theblade, which angle corresponds to the angle of inclination relative tothe axis 26 of the fibers of a sheet of fibers (not shown) constitutingthe structure of the blade.

The blade 23 further includes a second array of loops comprising foursubstantially rectilinear loops 10W, 10X, 10Y, and 10Z extending alongrespective mutually-parallel axes 19W, 19X, 19Y, and 19Z; these axes aresubstantially perpendicular to the axes 19A to 19C.

Thus, the loops of the first array cross the loops of the second array,with the second array being superposed on the first array.

The loops of the first array are disposed in a first layer or sheet,while the loops of the second array are disposed in a second layer orsheet that is separated from the first layer or sheet by a layer (notshown) of an electrically insulating material, in particular thethermoplastic or thermosetting resin impregnating the fibers of thecomposite structure of the blade 23.

As a result, no electrical contact exists at the intersections of twooverlying loops, such as the loops 10C and 10X.

Since the loops 10A to 10C and 10W to 10Z are identical, and since theirmutual spacing 27 measured along the axis 26 is much less than theproduct of their common length (20 in FIG. 2) multiplied by √2, eachloop of the first group or array crosses (in superposition) at least twoloops of the second group or array of loops.

By meshing the arrays of loops and resistor bridges in this way, in theevent of a break or an interruption occurring in the arrays in theshaded zone 28 of FIG. 3, the position and the approximate extent of thedamage to the blade can easily be determined.

Supposing that all the resistor bridges present an impedance of 1 kilohm(kQ), then such a break will open the loops 10C and 10Y in their middlezones, such that the impedances of these two loops will become 1 kQ,whereas the impedances of the other loops will retain the “normal” valueof about 333 ω.

As a function of the loop impedance measurements received from thesystem incorporated in the blade, an on-board computer controlled by aspecific damage-monitoring program including data representative of theconfiguration of the meshes stored in a memory of the computer, candetermine a maximum area 29 in which the damage must have occurred (andincluding the zone 28 that is actually damaged), corresponding to theimpedance measurements; it is thus easy not only to determine theposition of the damage, but also to determine an over-estimate of theextent of the damage.

With reference to FIGS. 3 to 5, each resistive loop is connected by apair of conductors 30 to a unit 34 integrated in the blade in thevicinity of the root zone 33 whereby the blade is connected (hinged) tothe rotor (not shown).

The conductors 30 are grouped together in a bundle 31 that extendslongitudinally (parallel to the axis 26) close to the leading edge 24 ofthe blade 23, and close to the spar 230 of the blade 23 (cf. FIG. 6).

In order to reduce the number of conductors, a common power supply wirecan be established for a plurality of adjacent loops or for all of theloops in an area, for example a pressure side or a suction side coveringof the blade.

The loops 10 extend from the bundle to the vicinity of the trailing edge25.

With reference to FIG. 4, the bundle 31 presents an end portion 31 a inthe vicinity of the free end 32 of the blade 23, which end portionfollows the angle of inclination of the leading edge in this location.

In the vicinity of the root 33 of the blade 23, resistive loops alsoextend between the bundle 31 and the leading edge 24.

In order to avoid overcrowding FIG. 3, the conductors 30 are shown forloops 10W to 10Z; nevertheless, one pair of conductors (not shown) isalso required for connecting each of the loops 10A to 10C to the unit 34(FIGS. 4 and 5); the unit includes a multiplexer 40 or the like forsuccessively scanning all of the loops of the blade, animpedance-measuring device 41 for measuring the impedance of each loop,and an analog-to-digital converter 42 for converting the measurementsignals delivered by the impedance-measuring device into digital data.

Connection means 43 (wired or wireless) serve to convey the impedancedata delivered by the converter 42 to the computer 44 on board theaircraft, for each of the blades of the aircraft rotor.

Where appropriate, impedance data for the loops can be conveyed and thecomponents housed in the unit on the blade can be powered via electricalconductors for delivering power and/or transporting data as already usedby some other system incorporated in the blade, e.g. a system forde-icing the blade.

A battery (not shown) incorporated in the unit 34 serves to power thecomponents 40 to 42.

The invention is particularly applicable to composite blades asdescribed in French patents Nos. 2 740 379, 2 699 498, and/or 2 699 499.

1. A rotor blade (23) for a rotary-wing aircraft, the blade presenting a longitudinal axis (26) and including (semi)conductive loops (10) extending along axes (19, 19A to 19C, 19W to 19Z) or directions that are oblique relative to the longitudinal axis (26), the blade further presenting a composite structure including fibers, at least some of the (semi)conductive loops extending parallel to the fibers.
 2. A blade according to claim 1, including a first group or array of first (semi)conductive loops extending in a first oblique axis or direction (19A to 19C), and a second group or array of second (semi)conductive loops extending in a second oblique or transverse axis or direction (19W to 19Z) that is distinct from the first axis (the first direction).
 3. A blade according to claim 2, in which the first direction is substantially perpendicular to the second direction.
 4. A blade according to claim 2, in which the groups or arrays of loops are separated by a layer of insulating material.
 5. A blade according to claim 1, including a first array of mutually-parallel loops disposed in a first layer and a second array of mutually-parallel loops crossing the first array.
 6. A blade according to claim 5, in which the arrays are superposed and separated by a layer of insulating material.
 7. A rotary-wing aircraft rotor blade (23) that presents a longitudinal axis (26) and that comprises: a composite structure including fibers; (semi)conductive loops (10) extending along axes (19, 19A to 19C, 19W to 19Z) or directions that are oblique relative to the longitudinal axis (26), at least some of the (semi)conductive loops extending parallel to the fibers; an impedance-measuring device (41) for measuring the impedance of a loop and delivering a loop impedance measurement signal; and a scanner and connector device (40) for connecting each of the loops in the plurality of loops in succession to the impedance-measuring device, with the loops being scanned regularly.
 8. A blade according to claim 7, in which each loop is connected to the scanner and connector device (40) via a pair of metal wires or strips (30).
 9. A blade according to claim 8, in which the metal wires or strips (30) extend close to the leading edge (24) of the blade.
 10. A blade according to claim 8, in which the metal wires or strips are grouped together in a bundle or cord (31).
 11. A blade according to claim 10, in which the loops extend from the bundle or cord (31) towards the trailing edge (25) of the blade.
 12. A blade according to claim 7, including an analog-to-digital converter (42) for transforming each measurement of the impedance-measuring device into digital impedance data, and transmission means (43) for transmitting the data to a computer (44).
 13. A blade according to claim 7, including, beside its pressure face or its suction face, a first group of loops extending in a first direction parallel to first fibers or to a first sheet of fibers of the structure of the composite blade, the blade further including a second group of loops extending in a second direction parallel to second fibers or to a second sheet of fibers of the structure of the composite blade, the second direction being distinct from the first direction.
 14. A rotary-wing aircraft rotor blade (23) that presents a longitudinal axis (26) and that is of composite structure including fibers, the blade including (semi)conductive loops (10) extending along axes (19, 19A to 19C, 19W to 19Z) or directions that are oblique relative to the longitudinal axis (26), at least some of the (semi)conductive loops extending parallel to the fibers, each loop comprising two strands and a plurality of conductive or semiconductive elements interconnecting the two strands and connected in parallel.
 15. A blade according to claim 14, in which each of the conductive or semiconductive elements presents substantially identical electrical impedance.
 16. A blade according to claim 14, in which each conductive or semiconductive element is constituted essentially by an SMC resistor.
 17. A blade according to claim 14, in which each loop includes at least three (semi)conductive elements regularly spaced apart from one another along the axis (19, 19A to 19C, 19W to 19Z) of the loop.
 18. A blade according to claim 14, and including several tens of (semi)conductive loops.
 19. A blade according to claim 14, in which the angle of inclination (22) of the loops relative to the longitudinal axis (26) of the blade is about 45°.
 20. A rotary-wing aircraft including a rotor having a plurality of blades, each blade comprising: a composite structure including fibers; and (semi)conductive loops (10) extending along axes (19, 19A to 19C, 19W to 19Z) or directions that are oblique relative to the longitudinal axis (26) of the blade, at least some of the (semi)conductive loops extending parallel to the fibers; the aircraft further comprising: an impedance-measuring device (41) for measuring the impedance of a loop and delivering a loop-impedance measurement signal; a scanner and connector device (40) for successively connecting each of the loops of a plurality of loops to the impedance-measuring device, with the loops being scanned regularly; a computer (44) associated with a memory containing data representative of the configuration of the meshing of the loops incorporated in each blade of the rotor; an analog-to-digital converter (42) for transforming each measurement of the impedance-measuring device into digital impedance data; and transmission means (43) for transmitting the data delivered by the converter to the computer (44); the computer being further associated with a program for determining an area (29) of a damage to a blade, the program being responsive to the impedance data and to the data representative of the configuration of the meshing of the loops of the blades. 